(1) Field of the Invention
The present invention relates to a process for preparing a unidirectional silicon steel sheet used for an iron core of electrical machinery and apparatus. The process of the present invention enables the preparation of a unidirectional silicon steel sheet having a high magnetic flux density.
(2) Description of the Prior Art
A unidirectional silicon steel sheet comprises grains having a Goss orientation wherein the steel plate surface has {1 1 0}face and the rolling direction has &lt;1 0 0&gt;axis ({1 1 0}&lt;0 0 1&gt;orientation in terms of Miller's indices), and is used as a soft magnetic material in an iron core of a transformer and a generator, and this steel sheet should have excellent magnetizing characteristics and iron loss characteristics, among the required magnetic characteristics. The magnetizing characteristics are determined by the magnetic flux density induced within the iron core in an applied given magnetic field, and in a product having a high magnetic flux density, the size of the iron core can be reduced. A high magnetic flux density can be attained by precisely orientating the steel plate grain to {1 1 0}&lt;0 0 1&gt;.
The iron loss is a power loss consumed as a thermal energy when a predetermined alternating current is applied to an iron core, and is influenced by the magnetic flux density, sheet thickness, amount of impurities, specific resistance, and size of grain, etc.
The steel sheet having a high magnetic flux density is preferred because not only can the size of the iron core of an electrical machinery and apparatus be reduced but also the iron loss becomes small. Therefore, there is a need in the art for the development of a process which enables a product having the possible highest magnetic flux density to be prepared at a low cost.
A unidirectional silicon steel plate is prepared by a secondary recrystallization, wherein a steel sheet prepared by subjecting a hot rolled sheet to a proper combination of cold rolling with annealing, to a final sheet thick.ness, is subjected to finishing annealing to selectively grow a primarily recrystallized grain having {110}&lt;001&gt;orientation. The secondary recrystallization is achieved when fine precipitates, e.g., MnS, AlN, MnSe, BN and (Al, Si)N, or elements present at grain boundaries, such as Sn and Sb, are present in the steel sheet before the secondary recrystallization. As described in J.B. May and D. Turnbull (Trans. Met. Soc. AIME 212 (1958), pp. 769-781), these precipitates and elements present at grain boundaries serve to selectively grow grains having {110}&lt;001&gt;orientation through suppression of the growth of primarily recrystallized grains having an orientation other than {110}&lt;001&gt; orientation in the step of finishing annealing. The above-described effect of suppressing the growth of grains is generally called the "inhibitor effect". Accordingly,, the main thrust of research and development in the art is toward the determining of what kind of precipitate or element present at grain boundaries should be used to stabilize the secondary recrystallization and how to achieve a proper state of existence of the above-described precipitate and element for enhancing the proportion of the existence of grains having an exact {110}&lt;001&gt;orientation. The method wherein use is made of only one precipitate has a limit on the control of {110}&lt;001&gt;orientation with a high accuracy. Therefore, in recent years, technical developments have been conducted to obtain a stable production of a product having a higher magnetic flux density, at a lower cost, through a thorough elucidation of the drawbacks and advantages of each precipitate, and an organic combination of several precipitates.
Currently, three representative processes for preparing a unidirectional silicon steel sheet on a commercial scale are known in the art, and each have advantages and disadvantages. The first technique is a double cold rolling process disclosed in Japanese Examined Patent Publication No. 30-3651 by M.F. Littmann wherein use is made of MnS. In this process, the resultant secondarily recrystallized grain is stably grown, but a high magnetic flux density is not obtained. The second technique is a process disclosed in Japanese Examined Patent Publication No. 40-15644 by Taguchi et al., wherein a combination of AlN with MnS is used to attain a draft as high as 80% or more in the final cold rolling. In this process, although a high magnetic flux density is obtained, a close control of production conditions is necessary for production of a commercial scale. The third technique is a process disclosed in Japanese Examined Patent Publication No. 51-13469 by Imanaka et al. wherein a silicon steel containing MnS (and/or MnSe) and Sb is produced by the double cold rolling process. In this process, although a relatively high magnetic flux density is obtained, the production cost becomes high due to the use of harmful and expensive elements, such as Sb and Se, and double cold rolling. The above-described three techniques have the three following problems in common. Specifically, in all of the above-described techniques, to finely and homogeneously control the precipitate, prior to the hot rolling, the slab is heated at a very high temperature, i.e., in the first technique at 1260.degree. C. or above, in the second technique at 1350.degree. C. when the silicon content is 3% although the temperature depends on the silicon content of the material as described in Japanese Unexamined Patent Publication No. 48-51852, in the third technique at 1230.degree. C. or above and 1320.degree. C. in the example wherein a high magnetic flux density is obtained as described in Japanese Unexamined Patent Publication No. 51-20716, thereby once melting the coarse precipitate to form a solid solution, and the precipitation is conducted during subsequent hot rolling or heat treatment. An increase in the slab heating temperature brings the problems of an increase in the energy used during heating of the slab, a lowering of the yield, and an increase in the repair cost of the heating furnace due to slag, a lowering in the operating efficiency attributable to an increase in the frequency of the repair of the heating furnace, and an inability to use a continuous cast slab due to occurrence of poor secondary recrystallization region in streak, recrystallization as described in Japanese Examined Patent Publication No. 57-41526. A more important consideration than the cost is that a large content of silicon and a thin product sheet thickness for a reduction of the iron loss brings an increase in the occurrence of the above-described poor secondary recrystallization region in streak, and thus a further reduction of the iron loss cannot be expected from the technique using the high temperature slab heating method. On the other hand, in the technique disclosed in Japanese Examined Patent Publication No. 61-60896, the sulfur content of the steel is reduced to stabilize the secondary recrystallization, which enables a product having a high silicon content and a small thickness to be prepared. Neverthe.less, when the production on a commercial scale is taken into consideration, this technique has a problem with regard to the stability of the magnetic flux density, and accordingly, an improved technique was proposed as described in, for example, Japanese Unexamined Patent Publication No. 62-40315. To date, however, a satisfactory solution to the above problem has not been found.
As described above, in the current industrial process, an inhibitor necessary for the secondary recrystallization is added in the step prior to cold rolling. By contrast, the present invention relates to a process based on the same technical concept as that disclosed in Japanese Unexamined Patent Publication No. 62-40315. Specifically, the inhibitor necessary for the secondary recrystallization is formed in situ between after the completion of the decarburizing annealing (primary recrystallization) and before the development of the secondary recrystallization in the finishing annealing. This is achieved by infiltrating nitrogen into the steel to form (Al, Si)N serving as an inhibitor. The infiltration of nitrogen may be conducted by the prior art method wherein the infiltration of nitrogen from the atmosphere in the step of increasing the temperature during finishing annealing is utilized or a strip is exposed to a gas atmosphere capable of serving as a nitriding atmosphere, such as NH , in the post-region of the decarburizing annealing or after the completion of decarburizing annealing.
To homogenize the nitriding treatment, an attempt has been made to carry out a nitriding treatment of a steel in the form of a loose strip coil. This method, however, is still unsatisfactory because problems arise such as a heterogeneous nitriding and unstable glass coating, depending upon conditions such as the surface state of the steel sheet, properties of the annealing separator, and additives.